Move constructor: move-constructs the first base class basic_istream as basic_istream<CharT,Traits>(std::move(rhs));, which in turn move-constructs and initializes the virtual base std::basic_ios. The initialization of the other base, basic_ostream, is implementation-defined (e.g., a protected default constructor may be added to std::basic_ostream, which does nothing) because move-construction cannot use rhs twice. This move constructor is protected: it is called by the move constructors of the derived stream classes std::basic_fstream and std::basic_stringstream before they move-construct and associate the stream buffer.

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